Though present in natural settings at minute quantities, ethylene oxide was first synthesized in a laboratory setting in 1859 by French chemist Charles-Adolphe Wurtz using the so-called “chlorohydrin” process. However, the usefulness of ethylene oxide as an industrial chemical was not fully understood in Wurtz's time; and so industrial production of ethylene oxide using the chlorohydrin process did not begin until the eve of the First World War due at least in part to the rapid increase in demand for ethylene glycol (of which ethylene oxide is an intermediate) as an antifreeze for use in the rapidly growing automobile market. Even then, the chlorohydrin process produced ethylene oxide in relatively small quantities and was highly uneconomical.
The chlorohydrin process was eventually supplanted by another process, the direct catalytic oxidation of ethylene with oxygen, the result of a second breakthrough in ethylene oxide synthesis, discovered in 1931 by another French chemist Thèodore Lefort. Lefort used a solid silver catalyst with a gas phase feed that included ethylene and utilized air as a source of oxygen. In the eighty years since the development of the direct oxidation method, the production of ethylene oxide has increased so significantly that today it is one of the largest volume products of the chemicals industry, accounting, by some estimates, for as much as half of the total value of organic chemicals produced by heterogeneous oxidation. Worldwide production in the year 2000 was about 15 billion tons. Almost 65% of ethylene oxide is hydrolyzed to MEG, which is a precursor for important polymers such as polyester fibers and polyethylene terephthalate. The second largest market for EO is in surface active agents, primarily non-ionic alkylphenol ethoxylates and detergent alcohol ethoxylates. Ethylene oxide itself can be polymerized to form polyethylene glycol or polyethylene oxide, which are useful as non-toxic, water-soluble polymers. Ethylene oxide gas kills bacteria, mold, and fungi, and can therefore is used to sterilize substances that would be damaged by sterilizing techniques such as pasteurization that rely on heat. Additionally, ethylene oxide is widely used to sterilize medical supplies such as bandages, sutures, and surgical implements. Monoethanolamine is produced by reacting ethylene oxide with aqueous ammonia; the reaction also produces diethanolamine and triethanolamine.
Given ethylene oxide's importance in industrial chemistry it is not surprising that a wide variety of catalysts have been formulated for the manufacture of ethylene oxide, each catalyst having its own unique characteristics. The “standard” ethylene oxide catalyst that has been in longest use contains primarily silver and Cs deposited on a low surface area carrier. The performance of this catalyst, which is often referred to as “high activity catalyst” is expected to be stable and with starting selectivity of 80-83%. This high activity catalyst does not require any special treatment in order to provide the expected performance. In the last twenty years, however, research has focused not on high activity catalysts, but on the so-called “high selectivity catalysts”, which are also Ag-based epoxidation catalysts. Among the high selectivity catalysts are those which contain small amounts of rhenium and cesium and other “promoting” elements. At optimum conditions, this type of catalyst provides selectivity in excess of 83% and can reach in excess of 90%.
With respect to these Re-containing catalysts there has been considerable interest in determining the optimum start-up (also commonly referred to as “initiation” or “activation”) conditions, since Re-containing catalysts require an initiation period to maximize selectivity. Among some of the earliest disclosures of initiation procedures for high selectivity catalysts are those in U.S. Pat. No. 4,874,879 to Lauritzen et al. and U.S. Pat. No. 5,155,242 to Shanker et al., which disclose start-up processes in which Re-containing catalysts are pre-chlorinated prior to the introduction of oxygen into the feed and the catalysts are allowed to “pre-soak” in the presence of the chloride at a temperature below that of the operating temperature. While some improvement in overall catalyst performance has been reported using these methods, the pre-soaking nonetheless was not sufficient to provide the optimum performance of the Re-containing catalyst. Additionally, in order to reduce the deleterious effects on catalyst performance caused by overchloriding during the pre-soak phase, occasionally it is necessary to conduct an additional chlorine removal step.
More recent techniques for initiation or conditioning procedures are disclosed in U.S. Pat. No. 7,485,597 to Lockemeyer et al. In particular, this disclosure provides a method for improving the selectivity of a supported highly selective epoxidation catalyst comprising Ag in a quantity of at most 0.17 g per m2 surface area of the support. The improvement was achieved by contacting the catalyst with a feed including oxygen at a catalyst temperature above 250° C. for duration of up to, at most, 150 hours.
Another start-up process is disclosed in U.S. Pat. No. 7,102,022 to Lockemeyer et al. Specifically, the '022 patent discloses a method for the start-up of a process for the epoxidation of an olefin comprising an Ag-based highly selectivity epoxidation catalyst. The method disclosed in the '022 patent includes contacting a catalyst bed with a feed comprising oxygen. In this treatment, the temperature of the catalyst bed was above 260° C. for a period of time of, at most, 150 hours.
U.S. Pat. No. 7,553,980 discloses an “initiation” method for epoxidation of ethylene comprising: contacting a catalyst bed including a silver-based highly selective epoxidation with a feed gas composition, at a first temperature during an initiation period, including ethylene, oxygen, a moderator and carbon dioxide, said carbon dioxide is at first concentration of greater than about 6 vol. %; increasing the first temperature to a second temperature, adjusting the feed gas composition in order to maintain said desired concentration of ethylene oxide while achieving a desired catalyst work rate. At the end of the initiation period, the second temperature is lowered to a third temperature.
In a co-pending patent application “Method for Preparing an Epoxidation Catalyst”, U.S. Ser. No. 13/109,657 filed on the same date as the present application, a two-step calcination is disclosed that eliminates the need for the catalyst “initiation” or conditioning period. The treatment includes first heating the catalyst precursor in an inert atmosphere and then the calcination continues in an oxygen-containing atmosphere at a temperature from about 350° C. to about 450° C. for a time period of up to about 5 minutes.
While the treatment methods for activating a Re-containing epoxidation catalyst disclosed in the aforementioned prior publications may provide some improvement in catalyst performance, they also have several deficiencies, noted above. At the very least the delay in production caused by method for activating or conditioning and also the complicated details of the procedure. Given the importance for operating Ag-based highly selective catalysts under optimum performance conditions, there is a continued need to develop new and improved methods that can be used for start-up of a process for the epoxidation of olefins, especially ethylene.